A. Field of the Invention
The present invention relates to drill bits and drilling machines. More particularly, the invention relates to an improved drill bit which reduces wear on collets into which the bit may be inserted into and removed from.
B. Description of Background Art
Drill bits or "drills" as they are frequently referred to are employed in a wide range of applications. One industry which uses drills in large quantities is the printed circuit board manufacturing industry. Printed circuit boards are used in a wide variety of electronic and electrical equipment in the consumer, commercial, industrial and military markets. Holes are drilled through the board to receive the leads of electronic components.
Typical printed circuit boards used in relatively simple consumer electronic equipment may require the drilling of 100 to 5,000 holes, in sizes ranging from 0.004 inch to 0.250 inch in diameter. More complex electronic equipment may employ printed circuit boards which require drilling as many as 50,000 holes of various sizes through the board.
Because of the large numbers of holes of various sizes which must be drilled at precisely determined locations of a printed circuit board, automatic drilling machines are frequently employed for this purpose.
Typical automatic drilling machines utilize a spindle with attached chuck, usually of the collet type. The collet holds a drill bit, and revolves about a vertically oriented axis. A two-axis servomechanism drives the spindle to precisely determined locations in an X-Y coordinate plane.
Typically, a set of hole coordinates is pre-programmed on a storage medium such as a paper tape, magnetic tape or disc, or the like. The data from the storage medium are then loaded into a controller which provides drive signals to the drilling machine. Such machines are referred to generically as "N/C" (Numerical Control) machine tools.
After the spindle of the drilling machine is located precisely in a X-Y coordinate plane, the spindle is moved downwards in the Z direction along its longitudinal axis into contact with a printed circuit board, or a stack of boards, to drill holes at precisely determined locations on the board(s).
Now as stated above, typical printed circuit boards use holes of various sizes adapted to receive the conductive leads of various size electronic and electrical components. Accordingly, most automatic drilling machines are provided with a capability for automatically removing and re-fitting different size drill bits into the collet of the drilling spindle. Drill bit sizes constitute another set of data pre-programmed into the data storage medium loaded into the N/C controller. Drill bits are also changed automatically after a pre-determined number of holes have been drilled, replacing a dull drill bit with a freshly sharpened bit.
Typically, N/C drilling machines used for printed circuit board manufacture utilize a collet-type chuck. The collet consists of a longitudinally split bushing having an outwardly flared lower end, and an entrance bore a few thousandths of an inch larger than the outer diameter of the shank of the drill bit which the collet is adapted to receive. To install a drill bit in the collet, the spindle is positioned above the shank of a selected drill bit resiliently held vertically upright in a cylindrically-shaped, elastic gripper. A row of such grippers, each holding a different drill bit, is typically held in an elongated structure referred to as a tool pod.
With a collet positioned over the shank of a selected drill bit, the collet is moved longitudinally downwards into contact with the shank of the drill bit, and further downwards until the shank of the bit is inserted a pre-determined longitudinal distance within the bore of the collet. The collet segments are then allowed to move radially inwards into clamping contact with the outer circumferential surface of the shank. The spindle is then moved vertically upwards, pulling the selected drill bit from its tool pod gripper.
Usually, a very small clearance exists between the inner diameter of an opened collet and a drill bit shank. Thus, even a small angular deviation between the longitudinal axes of the collet and drill bit shank can cause the shank of the drill bit to scuff the inner surface of the collet. After a sufficient number of drill bit loading and unloading cycles, this scuffing action can cause the collet opening to become eccentric. That eccentricity can result in drilling quality problems such as oversize holes, drill breakage and drill wander.
Scuffing contact between a collet and drill bit shank can cause problems in addition to collet wear, as will now be described.
Drill bits of the type used to drill holes in printed circuit boards are frequently provided with an annular ring that tightly grips the shank of the drill bit. The lower face of the ring is positioned a precisely determined distance rearward from the point of the drill bit, thus providing means for precisely controlling the insertion depth of the drill bit through a stack of boards. An insertion depth-limiting ring of the type described also provides a convenient structure for elastic engagement by a tool pod gripper.
Grippers used with drill bits having insertion depth-limiting rings typically consist of a longitudinally split, cylindrical elastic bushing having an upwardly facing bore opening. The bore of the bushing is often fitted with an inwardly projecting annular flange to limit the depth of insertion of the drill bit ring into the tool pod gripper. Even slight wear of the plastic gripper bushing can cause misalignment between the axis of the drill bit shank and the collet axis. As a result of this misalignment, downward movement of the collet to engage the drill bit exerts a downward force on the shank of the bit. This downward force during tool change is typically about 20 pounds. With sufficient angular misalignment between the collet and the drill bit shank, a downward force may be exerted on the shank. That downward force may be sufficiently large to push the shank through the insertion depth-limiting ring.
With the above-described problems associated with engagement of drill bits with a collet in mind, an improved drill bit was conceived of in an effort to alleviate those problems.